I'm trying to configure ME for handle email for mydomain.comME will operate at the public IP defined by hq.mydomain.commydomain.com has an MX record which specifies hq.mydomain.commydomain.com has an SPF record that includes hq.mydomain.com

The SPF record is properly defined under mydomain.com. Thats not the issue.

The basic question is how to configure ME to identify the host as hq.mydomain.com in the SMTP exchange, rather than the default machinenane.mydomain.com

For mydomane.com our MX record points to hq.mydomain.com, which resolves to the public IP for the mail server. We also have RDNS configured for hq.mydomain.com All of thus is working perfectly with our previous mail server. The only question is how/where to specify hq.mydomain.com vs mydomain.com